Grain Boundary Interface States Research Articles

Modification of electron states in CdTe absorber due to a buffer layer in CdTe/CdS solar cells

By application of the ac admittance spectroscopy method, the defect state energy distributions were determined in CdTe incorporated in thin film solar cell structures concluded on ZnO, ZnSe, and ZnS buffer layers. Together with the Mott-Schottky analysis, the results revealed a strong modification of the defect density of states and the concentration of the uncompensated acceptors as influenced by the choice of the buffer layer. In the solar cells formed on ZnSe and ZnS, the Fermi level and the energy position of the dominant deep trap levels were observed to shift closer to the midgap of CdTe, suggesting the mid-gap states may act as recombination centers and impact the open-circuit voltage and the fill factor of the solar cells. For the deeper states, the broadening parameter was observed to increase, indicating fluctuations of the charge on a microscopic scale. Such changes can be attributed to the grain-boundary strain and the modification of the charge trapped at the grain-boundary interface states in polycrystalline CdTe.

Recombination properties of grain boundaries inpolycrystalline silicon under illumination

This paper provides a theoretical study ofrecombination at the grain boundary (GB) in polysilicon underoptical illumination. The model is based on the assumptions ofa Gaussian energy distribution of the GB interfacestates and the acceptor- and donor-like interface states,and unequal capture cross sections of these GBstates for electrons and holes in polycrystalline materials. Withthe condition that we consider the GB material asamorphous silicon, which means that the bandwidth is about1.5 eV, the dependence of the GB potential barrierheight ({V}g), interface and effective recombinationvelocities of minority carriers at the edge of space-chargeregions ({S}(0), {S}({w})) and interfacerecombination current density ({J}R(0)) on theillumination level is investigated. Furthermore, the influenceof different dopant concentration in the grain({N}d) on the parameters ({S}(0),{S}({w}), {J}R(0)) is alsoinvestigated. The results indicate that {S}(0),{S}({w}) and {J}R(0) increase withincreasing {N}d under identicalillumination levels (Δ{E}f).

Grain-boundary-controlled transport in GaN layers

An exponential dependence of the photoconductivity on the surface photovoltage at GaN layers is predicted theoretically and confirmed experimentally. The prediction is based on the assumption that the material is mainly an ordered polycrystal, consisting of columnar grains. Accordingly, transport is expected to be limited by potential barriers at the grain boundaries, arising from the charge trapped at grain-boundary defects. The observed exponential dependence provides evidence that strongly supports the model by establishing a direct link between the bulk conductivity and the surface potential barrier. The same model is shown to successfully explain several other defect-related findings as well. In recent years, technological breakthroughs in GaN growth, doping, and contacting technologies have resulted in numerous devices, notably the blue GaN-based laser. 1 Progress, however, is still challenged by a high density of defects. 2 A typical finding is a columnar grain structure often observed in cross-sectional micrographs. 2 The effect of grain boundaries on transport in other semiconductors, especially polycrystalline Si, has been extensively studied. 3 It is widely accepted that trapped charge at grain-boundary interface states leads to the formation of potential barriers and depletion regions, as shown schematically in Fig. 1. These potential barriers resist intergrain transport, a hypothesis successfully used for explaining Hall 4 and ion-implantation 5 experiments in GaN films. If this mechanism is in effect, it can inclusively explain several intriguing transport-related phenomena observed at GaN layers, such as: ~i! a persistent and nonexponentially decaying photoconductivity ~PC!, 6,7 ~ii! a huge ultraviolet gain of GaN-based photodetectors, 8,9 ~iii! in ion-implanted films, a highly superlinear decrease of film conductivity with increasing implanted ion dose, 5 and ~iv! an increasing mobility with increasing carrier concentration, rather than the usual decrease. 4 In this paper, we present more direct evidence for grain-boundary limited transport in GaN, using surface photovoltage and photoconductivity measurements. We then rationalize the above-mentioned transport phenomena. Assuming a thermionic emission over a barrier and a small applied bias ~i.e., an effective bias much smaller than the thermal voltage across each grain!, barrier-limited transport is characterized by an exponential dependence of the conductivity, s, on the average grain-boundary barrier height f gb 3,10

Influence of temperature and interface charge on the grain-boundary conductivity in acceptor-doped SrTiO3 ceramics

The temperature dependence of both the grain-boundary potential barrier height and the conductivity across the grain-boundary space-charge depletion layer in acceptor-doped SrTiO3 ceramics has been investigated by a numerical simulation technique. The underlying model is that of a back-to-back double Schottky barrier at the grain boundary. The influence of the amount of positively charged donorlike grain-boundary interface states on the charge transport behavior of the grain-boundary region is also discussed. An interpretation in terms of a defect chemistry model of the bulk and the space-charge depletion layer, which on both sides surrounds the grain-boundary core is presented. The temperature behavior of the potential barrier at the grain boundary can be divided into three different regimes: a linear regime, and subsequently, saturation and decreasing regimes. The theoretical explanation for this behavior is given. Two different spatial conductivity profiles at the grain boundaries have to be considered, which clearly can be identified by two different characteristic thermal activation energies of the effective grain-boundary conductivity. The barrier height itself is not equal to the the thermal activation energy of the effective grain-boundary conductivity. The electrical characteristics of the grain boundaries can be influenced deliberately by decorating the grain boundaries with suitable dopants.

Grain-boundary interface electron traps in commercial zinc oxide varistors

A systematic study of the electronic characteristics of grain-boundary interface states was performed on a commercial zinc oxide varistor material using zero-bias deep level transient spectroscopy, current-voltage, high-frequency capacitance, and infrared reflectance techniques. Interface states, which act as deep acceptors, were found to exist at 0.97±0.12 eV below the conduction-band edge. The capture cross section is approximately 4×10−15 cm2. Characteristics of the interface states were determined as a function of the voltage of the trap filling pulse, which was varied from 0.9 to 2.0 V per grain boundary. It is shown that the trap energy is independent of the voltage of the pulse, while the apparent trap density increases with increasing voltage from 1.9 to 7.4×1011 cm−2. The results presented here indicate that this interface level is monoenergetic.

Observation of a high-resistance to a low-resistance transition in a silicon bicrystal

The transition from a high-resistance to a low-resistance state has been observed in a silicon bicrystal. This phenomenon is attributed to the trapping of minority carriers at the grain-boundary interface states. In the low-resistance state, the dc resistance of different samples falls into a narrow range of values, and exhibits, in addition, a positive temperature coefficient.

Influence of illumination on the grain boundary recombination velocity in silicon

The variation with illumination of the grain boundary (GB) barrier height EB and of the effective recombination velocity Seff is calculated by means of a self-consistent procedure which takes into account the bending of the minority carrier quasi-Fermi level in the GB space-charge region and in the GB quasi-neutral region. The GB interface states have been assumed to be uniformly distributed in a half-filled band whose width and position in the band gap can vary. Seff is nearly proportional to exp(EB/kT), only when EB is sufficiently low. For high EB, Seff is limited by the thermal velocity. The influence of the density of interface states and the grain doping concentration has been studied. The experimental results obtained with Silso–Wacker polycrystalline silicon show that the grain boundaries present different behaviors.

PLhenomenological model of grain-boundary trapping states in polycrystalline silicon under optical illumination

We have developed a new phenomenological model of grain boundary (GB) recombination, carrier transport and electrostatics under assumptions of Gaussian energy distributions of GB interface states, and unequal capture cross-sections of these GB states for electrons and holes in polycrystalline materials. Calculations have been performed of the recombination current density and the recombination velocity at grain boundaries in polycrystalline silicon for four different energy distributions. The results indicated that the recombination velocity at the grain boundary strongly depends on the location of the trap energy level. The assumption of GB interface states with a discrete (δ-function) distribution has been found to be inappropriate.

Minority-carrier injection(dark) metal-polycrystalline silicon contacts

The dependence of minority-carrier injection current Jp on applied forward-bias voltage of metalpolysilicon contacts has been calculated within a general treatment of the recombination at grain boundaries. A uniform distribution of interface states in energy at the grain boundaries is assumed throughout. It is found approximately that Jp ̃ exp (q V/KT) for (grain-boundary) recombination limited by the supply of minority carriers, Jp ̃ exp (q V/KT) or exp (q V/2KT) for high and low grain-boundary interface state densities for recombination limited by the supply of majority carriers, and Jp ̃ exp (3q V/4KT) for intrinsic recombination (limited equally by holes and electrons). Electrostatic effects of grain-boundary interface states, resulting in a bias dependence of grain-boundary barrier height øg, are significant for interface state densities Nis > 1010–1011cm−2V−1. depending upon the doping within the grains. The effective diffusion length for the injected minority carriers also shows an appreciable bias dependence, especially for intrinsic recombination, and for majority-carrier limited recombination at high øg and low Nis.Minority-carrier injection dominates the dark current (as it does in a p-n junction) for Schottky-barrier height øb > 0.8 V, for typical grain size d≃10−3cm, interface state densities Nis≃1013cm−2V−1, and relatively low doping Nd≃1014cm−3.